A television broadcast/dynamic image distribution service, which performs digital-transmission of dynamic video contents via a public radio broadcast such as a terrestrial broadcast, a satellite broadcast, or via various wired/radio communication networks, is becoming popular.
Improvements in dynamic image compression techniques such as MPEG have made it possible to increase the number of simultaneously distributable channels, so that viewer's demands of for various distributions can be satisfied. In the meantime, the increase in the number of distribution channels raises such an issue for users that it is difficult to select viewing channels. To overcome such issue, it has become popular for a receiving device to load a function of supporting the user to make judgment on selecting the channels.
Automatic collection of the representative images (thumbnail) is a typical example of such supporting function. With this function, for example, dynamic image streams being distributed in other channels that are not being viewed by a user are received/decoded on the background, and representative images showing the contents thereof are created to be stored in a memory. The collected representative images may be displayed in a form of list on a display screen when the user executes a button operation, for example, and the viewer can easily make judgment on which channel to watch next based upon the information.
A simple method for performing a cyclic stream reception is to collect streams of all the channels that are designated as collection targets while changing the target channels to be received/decoded in adequate orders. This method is effective for a dynamic image system employing conventional analog signal transmission mode, and it is possible to collect the images without deteriorating immediacy of the representative images even in a case of using a receiving device that has only a small number of receivable channels. For example, in a case of using a receiving device that is only capable of receiving one channel at a time for collecting representative images for fifteen channels where dynamic image signals of 30 frame/sec are transmitted, it is possible to complete collection of the streams of all the thirty channels in one second or less even if it requires 1/30 sec. at the worst for frame synchronization when switching the channels. Even if an image of one second earlier is presented at the time of request to display the representative image of each channel, there is no significant inconvenience for the user to select the channels.
However, when a dynamic image is transmitted by using a coding method based on inter-frame prediction, e.g., MPEG, it is not always possible to immediately decode picture streams received after switching the channels. This is because the picture that is coded based on inter-frame prediction cannot be decoded to a proper image unless a decoded image of the picture to be referred for prediction has not been obtained. In this case, decoding after the receiving stream is switched can only be started at the timing when an intra-frame coded picture is received. Therefore, it takes time to collect the streams of all the collection-target channels. Thus, when collecting the representative images, for example, the immediacy of the obtained images becomes deteriorated.
Specifically, in a non one-segmental terrestrial digital broadcasting (ISDB-T) (see Non-Patent Document 1), dynamic image data with time resolution of 30 frame/sec (interlace or progressive) is coded with a MPEG-2 system, and it is multiplexed to data of TS (Transport System) format to be transmitted. Except for a special occasion such as a change of scenes, an intra-frame coded picture (I picture) that can be individually decoded is inserted once in 0.5 sec. Others are coded as inter-frame prediction coded pictures called P pictures or B pictures. Thus, stochastically, there is waiting time of about 0.5 sec. at the maximum and 0.25 sec. on the average from the point where the receiving channel is switched to the point where the stream of I picture is received. As a result, for collecting the streams from fifteen channels, for example, there is waiting time of about 7.5 sec. at the maximum and 3.75 sec. on the average. Therefore, it is possible that there requires time of over 8 sec. at the worst together with the time for decoding the I picture.
As another example, in one-segmental broadcasting for portable equipment of the same ISDB-T, dynamic image data with time resolution of 15 frame/sec (progressive) is coded with H. 264 system and transmitted. It is allowed to have a time interval of 5 sec. at the maximum for insertion of the I-picture. Therefore, even if there are only five channels to collect the representative images, it requires 25 sec. for receiving the streams of all the channels under the worst conditions.
Such issues are generated not only in the television broadcast using public radio waves. Similar issues occur even with a system that receives distribution of dynamic images from a network of the Internet, when the distribution server does not have a special function for transmitting representative images, and there is no synchronization between the channels regarding insertion of the I pictures. Due to the characteristic of the inter-frame prediction coding, it is unavoidable that the time for collecting the streams increases in proportional to the interval of inserting the I pictures.
Regarding periodicity of inserting the I pictures mentioned above, the actual transmission time and receiving time of the stream data is not uniform. The length of the stream data that is obtained by coding each picture fluctuates within a prescribed range.
As a related technique thereof, there is Patent Document 1 described below, for example.
Patent Document 1 discloses a video displaying device which displays program videos of a plurality of channels on a single screen in a pseudo manner. This video displaying device is capable of switcheably displaying program videos including intra-frame coded images and inter-frame coded images, which includes: a detecting device for detecting appearing timings of intra-frame coded images of each program video to be processing targets; a storage device for storing appearance information showing the detected appearing timings; a referring device for referring to the appearance information of a second program video from the storage device, when switching from a first program video to the second program video; and a switching device for switching the processing target from the first program video to the second program video so that display is switched at the appearing timing of the intra-frame coded image of the second program video, based on the referred appearance information.
The referring device refers to the appearance information of a plurality of program videos as the switching candidates to compare the time at which the intra-frame coded images of each program video appear so as to select the program video that can be switched in the shortest time as the second program video. In this manner, there is executed the processing for switching the program video at the appearing timing of the I picture that is the intra-frame coded image, so as to suppress generation of a display-incapable period as much as possible.
Further, prediction of the appearing time of the I picture is conducted in a following manner, for example. First, the channel is switched by every specific time on a multi-screen to display each program video to be multi-screen display processing targets. At this time, the appearing timings of the I pictures of each program video (for example, an appearing tendency of the I pictures (picture structure of GOP) and appearing time of the I picture appeared lastly) are detected, and the appearance information showing the appearing timings is stored to the storage device. When switching of the channels of a full round is completed and it is returned to the first channel, the appearing time of the I picture thereafter is predicted by referring to the appearance information stored to the storage device.
Specifically, for switching from a program A to a program B, the appearance information of the program B is referred from the storage device. Then, integer N that satisfies Expression 1 is calculated from “last I picture appearing time Bl” and “interval Bd of I pictures” contained in the appearance information and “current time (channel switching time) C0”.|Bl+Bd×N−C0|<Bd  Expression 1
Then, appearing time C1′ of the I picture is calculated with Expression 2 by using the calculated integer N.C1′=Bl+Bd×N  Expression 2
In a case where the channel is switched again to the same channel with a relatively short period as in the multi-screen display, there is only a small possibility of having a scene change and the like during the time from the previous display to a next display. Therefore, it is possible to predict the appearing time of the I picture with high accuracy by referring to the appearance information of the previous display. When there is shift generated in the appearing time of the I picture because of a scene change or the like, the appearance tendency and the appearing time are corrected and stored.    Patent Document 1: Japanese Unexamined Patent publication 2005-184457    Non-Patent Document 1: “Digital Broadcasting Textbook, Vol. 1, Revised version” Impress Communications, 2004, pp. 66-73, pp. 120-130